Polyphenols: The Colourful and Protective Compounds in Plants

1. What Exactly Are They?

Polyphenols are tiny substances found in plants. They are so small that you cannot see them. You do not find them individually but as a vast family — like a drawer full of different things. Some give berries their blue, red or purple colour. Others make tea taste more astringent. Others still give an apple peel a slightly bitter taste. The name sounds like the Greek word for 'many'. And 'phenol' is a specific structural group. So: many building blocks that belong together. The plant produces these substances itself. It needs them to cope with sunlight, weather and insects. They protect the plant and give it colour and flavour. When you eat fruit or vegetables, the polyphenols enter your gut. What they do there is an exciting question. Researchers have already learned a lot about them and are still amazed. Some polyphenols travel through the body and are transformed by gut bacteria. Others do not get far. That is why it is not easy to say what they do overall.

2. Where Do They Come From?

Polyphenols are found in many plants you know. In berries, grapes, apples and cherries. In onions, cabbage and beans. They are also present in tea, cocoa and nuts. Spices such as cinnamon or cloves contain particularly high amounts — though you only eat them in small quantities. The plant forms these substances in its leaves, in the peel and in the seeds. When the sun shines strongly or an insect starts nibbling, the polyphenols are on hand. That is why they are often located where the plant is most vulnerable: just under the skin or on the outside of the peel. They are, in a sense, the plant’s protective suit. For you, this means: eating an apple with the peel gives you more polyphenols than eating a peeled apple. No one needs to change their diet because of this — it is simply interesting to know where they are found.

3. What Are Researchers Investigating?

There are a great many studies on polyphenols, 959 in total. In 133 of these, researchers worked with humans. In 64, the investigations were controlled: one group received the substance containing polyphenols, another received something for comparison, and no one knew who received what. This makes it easier to see whether a change really came from the polyphenols. Researchers looked, for example, at whether such plant substances help memory, whether they influence diseases of the lungs or liver, and whether they affect blood sugar after meals. In one study, people with slightly elevated blood sugar drank tea made from mulberry leaves and water chestnut. Such results are indications. Whether they apply to all people is still open. And researchers still do not know exactly which substance from the large family does what. That is precisely one of the next questions.

4. An Experiment to Think About

Cut an apple in half and place a piece on the table. After a while, the cut surface turns brown. What is happening? When you cut, cells burst open and the substances from inside come into contact with the air. This changes the colour. You can observe this at home at your leisure. You might also notice that a tea bag left to steep for a long time becomes darker and tastes more astringent. But what happens in nature or in your gut is not exactly the same as what happens on the kitchen table. This is precisely the exciting question: what we see in the kitchen — how much of that also plays a role in the body? Researchers are trying to find this out step by step.

For adults and professionals: the detailed page on polyphenols